A judge today abruptly declared a mistrial in the case of a man accused of sexually assaulting a 92-year-old woman at a Crystal Lake nursing home where he worked.

Both sides had given their opening statements and the first witness had testified when the judge determined that defendant Angelo Bird must be given a new trial later if prosecutors intend to continue the case against him.

During their opening statements, McHenry County prosecutors alleged that Bird, now 25, assaulted the woman at Crystal Pines Rehabilitation and Health Care Center in September 2011, when he worked there as a certified nursing assistant.

But defense attorneys argued that Bird was coerced into writing and signing a false confession after a lengthy interrogation in the Crystal Lake Police Station.

Bird is charged with criminal sexual assault of the woman, who remained at the facility until November 2011 but has since died.

The trial was halted after the state’s first witness, U.S. Secret Service Agent Booker Wright, used the word “test” twice in answering questions about an interview he had conducted with Bird in October 2011 during the investigation of the alleged crime.

The word was an apparent reference to a polygraph test which had been administered to Bird but which was not supposed to be mentioned in front of the jury.

Defense attorney Jamie Wombacher caught the agent’s apparent slip-up and motioned for a new trial, which Judge Michael Feetterer granted.

The judge said he could not know for sure whether jurors heard the word or, if so, whether they understood it referred to a polygraph test. Polygraphs, also known as lie-detector tests, are generally not allowed to be admitted in court proceedings because of controversy over their reliability.

As the dismissed jurors gathered in the hallway outside the courtroom, many said they had not heard or understood the reference.

The trial has been rescheduled for November.

Before the mistrial, Assistant State’s Attorney Sharyl Eisenstein told jurors that the woman reported that Bird, whom the woman called “daddy long legs,” came into her room early one morning and assaulted her with his finger.

Other staff members who came to work later that morning said the woman, who suffered from dementia and “was completely dependent on staff ... was upset, crying and banging the table against the wall.”

Later that night, her daughter took her to the hospital where she was treated for a urinary tract infection.

A hospital nurse there said the woman was “coherent and alert, but was upset,” Eisenstein said.

But Wombacher, the defense attorney, said the woman suffered from psychosis and had “delusions of persecution,” a condition where she believed people were seeking to harm her.

Doctors also did not detect any tearing, bruising or physical trauma on the woman, Wombacher said.

She said the woman had suffered urinary tract infections often and that a doctor would testify for the defense that delusions of persecution are a common side effect in older patients with dementia who have such infections.

Bird had worked at the nursing home for about three years prior to the allegation and not before been accused of a similar act, Wombacher said.

Wombacher said that, at the next trial, witnesses will testify that the woman had made up stories and accusations against others during her stay at the nursing home.

But Eisenstein said that, during the October 2011 interview involving Wright, Bird wrote and signed a confession saying that he was “curious” and that it was a “spur of the moment decision” to assault the woman.

Bird’s attorney, however, contended that he was put through a lengthy interrogation, was tired and only confessed to what Wright and another investigator had been suggesting he had done.

Bird, who is now married and expecting a child, was suspended for a week following the incident while nursing home officials conducted an investigation. He was allowed to work there again until authorities concluded their investigation and he was arrested and charged with the crime about three weeks later, Wombacher said.